The regulatory mechanism of contraction in smooth muscle is thought to involve the reversible phosphorylation of the myosin molecule. A myosin light chain kinase phosphorylates the light chains of the myosin molecule in the presence of ca2 plus and thereby activates the contractile apparatus, and another enzyme, a myosin light chain phosphatase removes the phosphate groups in the absence of Ca2 plus and returns the contractile apparatus to an inactive state. Our research in general will be concerned with elucidating the molecular mechanism of this scheme, and in particular, we will concentrate on establishing several properties of the myosin light chain kinase. It is important to determine the rate of phosphorylation, using both the isolated myosin light chians and the whole myosin molecule, since this might determine the rate at which contraction can occur. Several factors, including the phosphorylation of the kinase molecule and ADP, might influence the rate of phosphorylation and these will also be evaluated. In order to determine how phosphorylation might affect the contractile activity of smooth muscle the above studies will be compared to parallel assays on actomyosin ATPase activity. It is hoped in this way to elucidate the role that the phosphorylation of myosin plays in the contraction and relaxation phases of smooth muscle.